This is what it looks like:
https://imgur.com/a/mriUwIt scared the hell out of me! Are these guys losing their minds?
It was reported as a bug and the response thus far is indeed underwhelming for such a severe issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1424977
It wasn't supposed to be visible on the addons page
I don't think that makes it better. Knowing that there's a way to get an addon installed invisibly is going to be more justification for paranoia.
There almost certainly is not a way to invisibly install add-ons, unless you are part of Mozilla, and, you know, making Firefox. If paranoia is your thing, it might be worth considering that Mozilla can do anything it wants inside Firefox core, all of it is "invisible" to you.
Yes and a big part of this entire issue is users deciding whether we can trust Mozilla with that power or not.